Recombinational cloning using engineered recombination sites

ABSTRACT

Recombinational cloning is provided by the use of nucleic acids, vectors and methods, in vitro and in vivo, for moving or exchanging segments of DNA molecules using engineered recombination sites and recombination proteins to provide chimeric DNA molecules that have the desired characteristic(s) and/or DNA segment(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims the benefitunder 35 U.S.C. § 120 of, U.S. application Ser. No. 10/162,879, filedJun. 6, 2002, which is a continuation of, and claims the benefit under35 U.S.C. § 120 of, U.S. application Ser. No. 09/432,085, filed Nov. 2,1999, which is a divisional of, and claims the benefit under 35 U.S.C. §120 of, U.S. application Ser. No. 09/233,493, filed Jan. 20, 1999 (nowU.S. Pat. No. 6,143,557), which is a continuation of, and claims thebenefit under 35 U.S.C. § 120 of, U.S. application Ser. No. 08/663,002,filed Jun. 7, 1996 (now U.S. Pat. No. 5,888,732), which is acontinuation-in-part of, and claims the benefit under 35 U.S.C. § 120of, U.S. application Ser. No. 08/486,139, filed Jun. 7, 1995 (nowabandoned), which applications are entirely incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to recombinant DNA technology. DNA andvectors having engineered recombination sites are provided for use in arecombinational cloning method that enables efficient and specificrecombination of DNA segments using recombination proteins. The DNAs,vectors and methods are useful for a variety of DNA exchanges, such assubcloning of DNA, in vitro or in vivo.

2. Related Art

Site specific recombinases. Site specific recombinases are enzymes thatare present in some viruses and bacteria and have been characterized tohave both endonuclease and ligase properties. These recombinases (alongwith associated proteins in some cases) recognize specific sequences ofbases in DNA and exchange the DNA segments flanking those segments. Therecombinases and associated proteins are collectively referred to as“recombination proteins” (see, e.g., Landy, A., Current Opinion inBiotechnology 3:699-707 (1993)).

Numerous recombination systems from various organisms have beendescribed. See, e.g., Hoess et al., Nucleic Acids Research 14(6):2287(1986); Abremski et al., J Biol. Chem. 261(1):391 (1986); Campbell, J.Bacteriol. 174(23):7495 (1992); Qian et al., J Biol. Chem. 267(11):7794(1992); Araki et al., J. Mol. Biol. 225(l):25 (1992); Maeser andKahnmann (1991) Mol. Gen. Genet. 230:170-176).

Many of these belong to the integrase family of recombinases (Argos etal. EMBO J. 5:433-440 (1986)). Perhaps the best studied of these are theIntegrase/att system from bacteriophage λ (Landy, A. Current Opinions inGenetics and Devel. 3:699-707 (1993)), the Cre/loxP system frombacteriophage P1 (Hoess and Abremski (1990) In Nucleic Acids andMolecular Biology, vol. 4. Eds.: Eckstein and Lilley, Berlin-Heidelberg:Springer-Verlag; pp. 90-109), and the FLP/FRT system from theSaccharomyces cerevisiae 2μ circle plasmid (Broach et al. Cell29:227-234 (1982)).

Backman (U.S. Pat. No. 4,673,640) discloses the in vivo use of λrecombinase to recombine a protein producing DNA segment by enzymaticsite-specific recombination using wild-type recombination sites attB andattP.

Hasan and Szybalski (Gene 56:145-151 (1987)) discloses the use of λ Intrecombinase in vivo for intramolecular recombination between wild typeattP and attB sites which flank a promoter. Because the orientations ofthese sites are inverted relative to each other, this causes anirreversible flipping of the promoter region relative to the gene ofinterest.

Palazzolo et al. Gene 88:25-36 (1990), discloses phage lambda vectorshaving bacteriophage λ arms that contain restriction sites positionedoutside a cloned DNA sequence and between wild-type loxP sites.Infection of E. coli cells that express the Cre recombinase with thesephage vectors results in recombination between the loxP sites and the invivo excision of the plasmid replicon, including the cloned cDNA.

Pósfai et al. (Nucl. Acids Res. 22:2392-2398 (1994)) discloses a methodfor inserting into genomic DNA partial expression vectors having aselectable marker, flanked by two wild-type FRT recognition sequences.FLP site-specific recombinase as present in the cells is used tointegrate the vectors into the genome at predetermined sites. Underconditions where the replicon is functional, this cloned genomic DNA canbe amplified.

Bebee et al. (U.S. Pat. No. 5,434,066) discloses the use ofsite-specific recombinases such as Cre for DNA containing two loxP sitesis used for in vivo recombination between the sites.

Boyd (Nucl. Acids Res. 21:817-821 (1993)) discloses a method tofacilitate the cloning of blunt-ended DNA using conditions thatencourage intermolecular ligation to a dephosphorylated vector thatcontains a wild-type loxP site acted upon by a Cre site-specificrecombinase present in E. coli host cells.

Waterhouse et al. (PCT No. 93/19172 and Nucleic Acids Res. 21 (9):2265(1993)) disclose an in vivo method where light and heavy chains of aparticular antibody were cloned in different phage vectors between loxPand loxP 511 sites and used to transfect new E. coli cells. Cre, actingin the host cells on the two parental molecules (one plasmid, onephage), produced four products in equilibrium: two differentcointegrates (produced by recombination at either loxP or loxP 511sites), and two daughter molecules, one of which was the desiredproduct.

In contrast to the other related art, Schlake & Bode (Biochemistry33:12746-12751 (1994)) discloses an in vivo method to exchangeexpression cassettes at defined chromosomal locations, each flanked by awild type and a spacer-mutated FRT recombination site. Adouble-reciprocal crossover was mediated in cultured mammalian cells byusing this FLP/FRT system for site-specific recombination.

Transposases. The family of enzymes, the transposases, has also beenused to transfer genetic information between replicons. Transposons arestructurally variable, being described as simple or compound, buttypically encode the recombinase gene flanked by DNA sequences organizedin inverted orientations. Integration of transposons can be random orhighly specific. Representatives such as Tn7, which are highlysite-specific, have been applied to the in vivo movement of DNA segmentsbetween replicons (Lucklow et al., J. Virol. 67:4566-4579 (1993)).

Devine and Boeke Nucl. Acids Res. 22:3765-3772 (1994), discloses theconstruction of artificial transposons for the insertion of DNAsegments, in vitro, into recipient DNA molecules. The system makes useof the integrase of yeast TY1 virus-like particles. The DNA segment ofinterest is cloned, using standard methods, between the ends of thetransposon-like element TY1. In the presence of the TY1 integrase, theresulting element integrates randomly into a second target DNA molecule.

DNA cloning. The cloning of DNA segments currently occurs as a dailyroutine in many research labs and as a prerequisite step in many geneticanalyses. The purpose of these clonings is various, however, two generalpurposes can be considered: (1) the initial cloning of DNA from largeDNA or RNA segments (chromosomes, YACs, PCR fragments, mRNA, etc.), donein a relative handful of known vectors such as pUC, pGem, pBlueScript,and (2) the subcloning of these DNA segments into specialized vectorsfor functional analysis. A great deal of time and effort is expendedboth in the initial cloning of DNA segments and in the transfer of DNAsegments from the initial cloning vectors to the more specializedvectors. This transfer is called subcloning.

The basic methods for cloning have been known for many years and havechanged little during that time. A typical cloning protocol is asfollows:

-   -   (1) digest the DNA of interest with one or two restriction        enzymes;    -   (2) gel purify the DNA segment of interest when known;    -   (3) prepare the vector by cutting with appropriate restriction        enzymes, treating with alkaline phosphatase, gel purify etc., as        appropriate;    -   (4) ligate the DNA segment to vector, with appropriate controls        to estimate background of uncut and self-ligated vector;    -   (5) introduce the resulting vector into an E. coli host cell;    -   (6) pick selected colonies and grow small cultures overnight;    -   (7) make DNA minipreps; and    -   (8) analyze the isolated plasmid on agarose gels (often after        diagnostic restriction enzyme digestions) or by PCR.

The specialized vectors used for subcloning DNA segments arefunctionally diverse. These include but are not limited to: vectors forexpressing genes in various organisms; for regulating gene expression;for providing tags to aid in protein purification or to allow trackingof proteins in cells; for modifying the cloned DNA segment (e.g.,generating deletions); for the synthesis of probes (e.g., riboprobes);for the preparation of templates for DNA sequencing; for theidentification of protein coding regions; for the fusion of variousprotein-coding regions; to provide large amounts of the DNA of interest,etc. It is common that a particular investigation will involvesubcloning the DNA segment of interest into several differentspecialized vectors.

As known in the art, simple subclonings can be done in one day (e.g.,the DNA segment is not large and the restriction sites are compatiblewith those of the subcloning vector). However, many other subcloningscan take several weeks, especially those involving unknown sequences,long fragments, toxic genes, unsuitable placement of restriction sites,high backgrounds, impure enzymes, etc. Subcloning DNA fragments is thusoften viewed as a chore to be done as few times as possible.

Several methods for facilitating the cloning of DNA segments have beendescribed, e.g., as in the following references.

Ferguson, J., et al. Gene 16:191 (1981), discloses a family of vectorsfor subcloning fragments of yeast DNA. The vectors encode kanamycinresistance. Clones of longer yeast DNA segments can be partiallydigested and ligated into the subcloning vectors. If the originalcloning vector conveys resistance to ampicillin, no purification isnecessary prior to transformation, since the selection will be forkanamycin.

Hashimoto-Gotoh, T., et al. Gene 41:125 (1986), discloses a subcloningvector with unique cloning sites within a streptomycin sensitivity gene;in a streptomycin-resistant host, only plasmids with inserts ordeletions in the dominant sensitivity gene will survive streptomycinselection.

Accordingly, traditional subcloning methods, using restriction enzymesand ligase, are time consuming and relatively unreliable. Considerablelabor is expended, and if two or more days later the desired subclonecan not be found among the candidate plasmids, the entire process mustthen be repeated with alternative conditions attempted. Although sitespecific recombinases have been used to recombine DNA in vivo, thesuccessful use of such enzymes in vitro was expected to suffer fromseveral problems. For example, the site specificities and efficiencieswere expected to differ in vitro; topologically-linked products wereexpected; and the topology of the DNA substrates and recombinationproteins was expected to differ significantly in vitro (see, e.g., Adamset al, J. Mol. Biol. 226:661-73 (1992)). Reactions that could go on formany hours in vivo were expected to occur in significantly less time invitro before the enzymes became inactive. Multiple DNA recombinationproducts were expected in the biological host used, resulting inunsatisfactory reliability, specificity or efficiency of subcloning. Invitro recombination reactions were not expected to be sufficientlyefficient to yield the desired levels of product.

Accordingly, there is a long felt need to provide an alternativesubcloning system that provides advantages over the known use ofrestriction enzymes and ligases.

SUMMARY OF THE INVENTION

The present invention provides nucleic acid, vectors and methods forobtaining chimeric nucleic acid using recombination proteins andengineered recombination sites, in vitro or in vivo. These methods arehighly specific, rapid, and less labor intensive than what is disclosedor suggested in the related background art. The improved specificity,speed and yields of the present invention facilitates DNA or RNAsubcloning, regulation or exchange useful for any related purpose. Suchpurposes include in vitro recombination of DNA segments and in vitro orin vivo insertion or modification of transcribed, replicated, isolatedor genomic DNA or RNA.

The present invention relates to nucleic acids, vectors and methods formoving or exchanging segments of DNA using at least one engineeredrecombination site and at least one recombination protein to providechimeric DNA molecules which have the desired characteristic(s) and/orDNA segment(s). Generally, one or more parent DNA molecules arerecombined to give one or more daughter molecules, at least one of whichis the desired Product DNA segment or vector. The invention thus relatesto DNA, RNA, vectors and methods to effect the exchange and/or to selectfor one or more desired products.

One embodiment of the present invention relates to a method of makingchimeric DNA, which comprises

(a) combining in vitro or in vivo

-   -   (i) an Insert Donor DNA molecule, comprising a desired DNA        segment flanked by a first recombination site and a second        recombination site, wherein the first and second recombination        sites do not recombine with each other;    -   (ii) a Vector Donor DNA molecule containing a third        recombination site and a fourth recombination site, wherein the        third and fourth recombination sites do not recombine with each        other; and    -   (iii) one or more site specific recombination proteins capable        of recombining the first and third recombinational sites and/or        the second and fourth recombinational sites;

thereby allowing recombination to occur, so as to produce at least oneCointegrate DNA molecule, at least one desired Product DNA moleculewhich comprises said desired DNA segment, and optionally a Byproduct DNAmolecule; and then, optionally,

(b) selecting for the Product or Byproduct DNA molecule.

Another embodiment of the present invention relates to a kit comprisinga carrier or receptacle being compartmentalized to receive and holdtherein at least one container, wherein a first container contains a DNAmolecule comprising a vector having at least two recombination sitesflanking a cloning site or a Selectable marker, as described herein. Thekit optionally further comprises:

-   -   (i) a second container containing a Vector Donor plasmid        comprising a subcloning vector and/or a Selectable marker of        which one or both are flanked by one or more engineered        recombination sites; and/or    -   (ii) a third container containing at least one recombination        protein which recognizes and is capable of recombining at least        one of said recombination sites.

Other embodiments include DNA and vectors useful in the methods of thepresent invention. In particular, Vector Donor molecules are provided inone embodiment, wherein DNA segments within the Vector Donor areseparated either by, (i) in a circular Vector Donor, at least tworecombination sites, or (ii) in a linear Vector Donor, at least onerecombination site, where the recombination sites are preferablyengineered to enhance specificity or efficiency of recombination.

One Vector Donor embodiment comprises a first DNA segment and a secondDNA segment, the first or second segment comprising a Selectable marker.A second Vector Donor embodiment comprises a first DNA segment and asecond DNA segment, the first or second DNA segment comprising a toxicgene. A third Vector Donor embodiment comprises a first DNA segment anda second DNA segment, the first or second DNA segment comprising aninactive fragment of at least one Selectable marker, wherein theinactive fragment of the Selectable marker is capable of reconstitutinga functional Selectable marker when recombined across the first orsecond recombination site with another inactive fragment of at least oneSelectable marker.

The present recombinational cloning method possesses several advantagesover previous in vivo methods. Since single molecules of recombinationproducts can be introduced into a biological host, propagation of thedesired Product DNA in the absence of other DNA molecules (e.g.,starting molecules, intermediates, and by-products) is more readilyrealized. Reaction conditions can be freely adjusted in vitro tooptimize enzyme activities. DNA molecules can be incompatible with thedesired biological host (e.g., YACs, genomic DNA, etc.), can be used.Recombination proteins from diverse sources can be employed, together orsequentially.

Other embodiments will be evident to those of ordinary skill in the artfrom the teachings contained herein in combination with what is known tothe art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts one general method of the present invention, wherein thestarting (parent) DNA molecules can be circular or linear. The goal isto exchange the new subcloning vector D for the original cloning vectorB. It is desirable in one embodiment to select for AD and against allthe other molecules, including the Cointegrate. The square and circleare sites of recombination: e.g., loxP sites, att sites, etc. Forexample, segment D can contain expression signals, new drug markers, neworigins of replication, or specialized functions for mapping orsequencing DNA.

FIG. 2A depicts an in vitro method of recombining an Insert Donorplasmid (here, pEZC705) with a Vector Donor plasmid (here, pEZC726), andobtaining Product DNA and Byproduct daughter molecules. The tworecombination sites are attP and loxP on the Vector Donor. On onesegment defined by these sites is a kanamycin resistance gene whosepromoter has been replaced by the tetOP operator/promoter fromtransposon Tn10. See Sizemore et al., Nucl. Acids Res. 18(10):2875(1990). In the absence of tet repressor protein, E. coli RNA polymerasetranscribes the kanamycin resistance gene from the tetOP. If tetrepressor is present, it binds to tetOP and blocks transcription of thekanamycin resistance gene. The other segment of pEZC726 has the tetrepressor gene expressed by a constitutive promoter. Thus cellstransformed by pEZC726 are resistant to chloramphenicol, because of thechloramphenicol acetyl transferase gene on the same segment as tetR, butare sensitive to kanamycin. The recombinase-mediated reactions result inseparation of the tetR gene from the regulated kanamycin resistancegene. This separation results in kanamycin resistance in cells receivingonly the desired recombination products. The first recombinationreaction is driven by the addition of the recombinase called Integrase.The second recombination reaction is driven by adding the recombinaseCre to the Cointegrate (here, pEZC7 Cointegrate).

FIG. 2B depicts a restriction map of pEZC705.

FIG. 2C depicts a restriction map of pEZC726.

FIG. 2D depicts a restriction map of pEZC7 Cointegrate.

FIG. 2E depicts a restriction map of Intprod.

FIG. 2F depicts a restriction map of Intbypro.

FIG. 3A depicts an in vitro method of recombining an Insert Donorplasmid (here, pEZC602) with a Vector Donor plasmid (here, pEZC629), andobtaining Product (here, EZC6prod) and Byproduct (here, EZC6Bypr)daughter molecules. The two recombination sites are loxP and loxP 511.One segment of pEZC629 defined by these sites is a kanamycin resistancegene whose promoter has been replaced by the tetOP operator/promoterfrom transposon Tn10. In the absence of tet repressor protein, E. coliRNA polymerase transcribes the kanamycin resistance gene from the tetOP.If tet repressor is present, it binds to tetOP and blocks transcriptionof the kanamycin resistance gene. The other segment of pEZC629 has thetet repressor gene expressed by a constitutive promoter. Thus cellstransformed by pEZC629 are resistant to chloramphenicol, because of thechloramphenicol acetyl transferase gene on the same segment as tetR, butare sensitive to kanamycin. The reactions result in separation of thetetR gene from the regulated kanamycin resistance gene. This separationresults in kanamycin resistance in cells receiving the desiredrecombination product. The first and the second recombination events aredriven by the addition of the same recombinase, Cre.

FIG. 3B depicts a restriction map of EZC6Bypr.

FIG. 3C depicts a restriction map of EZC6prod.

FIG. 3D depicts a restriction map of pEZC602.

FIG. 3E depicts a restriction map of pEZC629.

FIG. 3F depicts a restriction map of EZC6coint.

FIG. 4A depicts an application of the in vitro method of recombinationalcloning to subclone the chloramphenicol acetyl transferase gene into avector for expression in eukaryotic cells. The Insert Donor plasmid,pEZC843, is comprised of the chloramphenicol acetyl transferase gene ofE. coli, cloned between loxP and attB sites such that the loxP site ispositioned at the 5′-end of the gene. The Vector Donor plasmid,pEZC1003, contains the cytomegalovirus eukaryotic promoter apposed to aloxP site. The supercoiled plasmids were combined with lambda Integraseand Cre recombinase in vitro. After incubation, competent E. coli cellswere transformed with the recombinational reaction solution. Aliquots oftransformations were spread on agar plates containing kanamycin toselect for the Product molecule (here CMVProd).

FIG. 4B depicts a restriction map of pEZC843.

FIG. 4C depicts a restriction map of pEZC1003.

FIG. 4D depicts a restriction map of CMVBypro.

FIG. 4E depicts a restriction map of CMVProd.

FIG. 4F depicts a restriction map of CMVcoint.

FIG. 5A depicts a vector diagram of pEZC1301.

FIG. 5B depicts a vector diagram of pEZC1305.

FIG. 5C depicts a vector diagram of pEZC1309.

FIG. 5D depicts a vector diagram of pEZC1313.

FIG. 5E depicts a vector diagram of pEZC1317.

FIG. 5F depicts a vector diagram of pEZC1321.

FIG. 5G depicts a vector diagram of pEZC1405.

FIG. 5H depicts a vector diagram of pEZC1502.

FIG. 6A depicts a vector diagram of pEZC1603.

FIG. 6B depicts a vector diagram of pEZC1706.

FIG. 7A depicts a vector diagram of pEZC2901.

FIG. 7B depicts a vector diagram of pEZC2913

FIG. 7C depicts a vector diagram of pEZC3101.

FIG. 7D depicts a vector diagram of pEZC1802.

FIG. 8A depicts a vector diagram of pGEX-2TK.

FIG. 8B depicts a vector diagram of pEZC3501.

FIG. 8C depicts a vector diagram of pEZC3601.

FIG. 8D depicts a vector diagram of pEZC3609.

FIG. 8E depicts a vector diagram of pEZC3617.

FIG. 8F depicts a vector diagram of pEZC3606.

FIG. 8G depicts a vector diagram of pEZC3613.

FIG. 8H depicts a vector diagram of pEZC3621.

FIG. 8I depicts a vector diagram of GST-CAT.

FIG. 8J depicts a vector diagram of GST-phoA.

FIG. 8K depicts a vector diagram of pEZC3201.

DETAILED DESCRIPTION OF THE INVENTION

It is unexpectedly discovered in the present invention that subcloningreactions can be provided using recombinational cloning. Recombinationcloning according to the present invention uses DNAs, vectors andmethods, in vitro and in vivo, for moving or exchanging segments of DNAmolecules using engineered recombination sites and recombinationproteins. These methods provide chimeric DNA molecules that have thedesired characteristic(s) and/or DNA segment(s).

The present invention thus provides nucleic acid, vectors and methodsfor obtaining chimeric nucleic acid using recombination proteins andengineered recombination sites, in vitro or in vivo. These methods arehighly specific, rapid, and less labor intensive than what is disclosedor suggested in the related background art. The improved specificity,speed and yields of the present invention facilitates DNA or RNAsubcloning, regulation or exchange useful for any related purpose. Suchpurposes include in vitro recombination of DNA segments and in vitro orin vivo insertion or modification of transcribed, replicated, isolatedor genomic DNA or RNA.

Definitions

In the description that follows, a number of terms used in recombinantDNA technology are utilized extensively. In order to provide a clear andconsistent understanding of the specification and claims, including thescope to be given such terms, the following definitions are provided.

Byproduct: is a daughter molecule (a new clone produced after the secondrecombination event during the recombinational cloning process) lackingthe DNA which is desired to be subcloned.

Cointegrate: is at least one recombination intermediate DNA molecule ofthe present invention that contains both parental (starting) DNAmolecules. It will usually be circular. In some embodiments it can belinear.

Host: is any prokaryotic or eukaryotic organism that can be a recipientof the recombinational cloning Product. A “host,” as the term is usedherein, includes prokaryotic or eukaryotic organisms that can begenetically engineered. For examples of such hosts, see Maniatis et al.,Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory,Cold Spring Harbor, N.Y. (1982).

Insert: is the desired DNA segment (segment A of FIG. 1) which onewishes to manipulate by the method of the present invention. The insertcan have one or more genes.

Insert Donor: is one of the two parental DNA molecules of the presentinvention which carries the Insert. The Insert Donor DNA moleculecomprises the Insert flanked on both sides with recombination signals.The Insert Donor can be linear or circular. In one embodiment of theinvention, the Insert Donor is a circular DNA molecule and furthercomprises a cloning vector sequence outside of the recombination signals(see FIG. 1).

Product: is one or both the desired daughter molecules comprising the Aand D or B and C sequences which are produced after the secondrecombination event during the recombinational cloning process (see FIG.1). The Product contains the DNA which was to be cloned or subcloned.

Promoter: is a DNA sequence generally described as the 5′-region of agene, located proximal to the start codon. The transcription of anadjacent DNA segment is initiated at the promoter region. A repressiblepromoter's rate of transcription decreases in response to a repressingagent. An inducible promoter's rate of transcription increases inresponse to an inducing agent. A constitutive promoter's rate oftranscription is not specifically regulated, though it can vary underthe influence of general metabolic conditions.

Recognition sequence: Recognition sequences are particular DNA sequenceswhich a protein, DNA, or RNA molecule (e.g., restriction endonuclease, amodification methylase, or a recombinase) recognizes and binds. Forexample, the recognition sequence for Cre recombinase is loxP which is a34 base pair sequence comprised of two 13 base pair inverted repeats(serving as the recombinase binding sites) flanking an 8 base pair coresequence. See FIG. 1 of Sauer, B., Current Opinion in Biotechnology5:521-527 (1994). Other examples of recognition sequences are the attB,attP, attL, and attR sequences which are recognized by the recombinaseenzyme λ Integrase. attB is an approximately 25 base pair sequencecontaining two 9 base pair core-type Int binding sites and a 7 base pairoverlap region. attP is an approximately 240 base pair sequencecontaining core-type Int binding sites and arm-type Int binding sites aswell as sites for auxiliary proteins IHF, FIS, and Xis. See Landy,Current Opinion in Biotechnology 3:699-707 (1993). Such sites are alsoengineered according to the present invention to enhance methods andproducts.

Recombinase: is an enzyme which catalyzes the exchange of DNA segmentsat specific recombination sites.

Recombinational Cloning: is a method described herein, whereby segmentsof DNA molecules are exchanged, inserted, replaced, substituted ormodified, in vitro or in vivo.

Recombination proteins: include excisive or integrative proteins,enzymes, co-factors or associated proteins that are involved inrecombination reactions involving one or more recombination sites. See,Landy (1994), infra.

Repression cassette: is a DNA segment that contains a repressor of aSelectable marker present in the subcloning vector.

Selectable marker: is a DNA segment that allows one to select for oragainst a molecule or a cell that contains it, often under particularconditions. These markers can encode an activity, such as, but notlimited to, production of RNA, peptide, or protein, or can provide abinding site for RNA, peptides, proteins, inorganic and organiccompounds or compositions and the like. Examples of Selectable markersinclude but are not limited to: (1) DNA segments that encode productswhich provide resistance against otherwise toxic compounds (e.g.,antibiotics); (2) DNA segments that encode products which are otherwiselacking in the recipient cell (e.g., tRNA genes, auxotrophic markers);(3) DNA segments that encode products which suppress the activity of agene product; (4) DNA segments that encode products which can be readilyidentified (e.g., phenotypic markers such as β-galactosidase, greenfluorescent protein (GFP), and cell surface proteins); (5) DNA segmentsthat bind products which are otherwise detrimental to cell survivaland/or function; (6) DNA segments that otherwise inhibit the activity ofany of the DNA segments described in Nos. 1-5 above (e.g., antisenseoligonucleotides); (7) DNA segments that bind products that modify asubstrate (e.g. restriction endonucleases); (8) DNA segments that can beused to isolate a desired molecule (e.g. specific protein bindingsites); (9) DNA segments that encode a specific nucleotide sequencewhich can be otherwise non-functional (e.g., for PCR amplification ofsubpopulations of molecules); and/or (10) DNA segments, which whenabsent, directly or indirectly confer sensitivity to particularcompounds.

Selection scheme: is any method which allows selection, enrichment, oridentification of a desired Product or Product(s) from a mixturecontaining the Insert Donor, Vector Donor, and/or any intermediates,(e.g. a Cointegrate) Byproducts. The selection schemes of one preferredembodiment have at least two components that are either linked orunlinked during recombinational cloning. One component is a Selectablemarker. The other component controls the expression in vitro or in vivoof the Selectable marker, or survival of the cell harboring the plasmidcarrying the Selectable marker. Generally, this controlling element willbe a repressor or inducer of the Selectable marker, but other means forcontrolling expression of the Selectable marker can be used. Whether arepressor or activator is used will depend on whether the marker is fora positive or negative selection, and the exact arrangement of thevarious DNA segments, as will be readily apparent to those skilled inthe art. A preferred requirement is that the selection scheme results inselection of or enrichment for only one or more desired Products. Asdefined herein, to select for a DNA molecule includes (a) selecting orenriching for the presence of the desired DNA molecule, and (b)selecting or enriching against the presence of DNA molecules that arenot the desired DNA molecule.

In one embodiment, the selection schemes (which can be carried outreversed) will take one of three forms, which will be discussed in termsof FIG. 1. The first, exemplified herein with a Selectable marker and arepressor therefor, selects for molecules having segment D and lackingsegment C. The second selects against molecules having segment C and formolecules having segment D. Possible embodiments of the second formwould have a DNA segment carrying a gene toxic to cells into which thein vitro reaction products are to be introduced. A toxic gene can be aDNA that is expressed as a toxic gene product (a toxic protein or RNA),or can be toxic in and of itself. (In the latter case, the toxic gene isunderstood to carry its classical definition of “heritable trait”.)

Examples of such toxic gene products are well known in the art, andinclude, but are not limited to, restriction endonucleases (e.g., DpnI)and genes that kill hosts in the absence of a suppressing function,e.g., kicB. A toxic gene can alternatively be selectable in vitro, e.g.,a restriction site.

In the second form, segment D carries a Selectable marker. The toxicgene would eliminate transformants harboring the Vector Donor,Cointegrate, and Byproduct molecules, while the Selectable marker can beused to select for cells containing the Product and against cellsharboring only the Insert Donor.

The third form selects for cells that have both segments A and D in cison the same molecule, but not for cells that have both segments in transon different molecules. This could be embodied by a Selectable markerthat is split into two inactive fragments, one each on segments A and D.

The fragments are so arranged relative to the recombination sites thatwhen the segments are brought together by the recombination event, theyreconstitute a functional Selectable marker. For example, therecombinational event can link a promoter with a structural gene, canlink two fragments of a structural gene, or can link genes that encode aheterodimeric gene product needed for survival, or can link portions ofa replicon.

Site-specific recombinase: is a type of recombinase which typically hasat least the following four activities: (1) recognition of one or twospecific DNA sequences; (2) cleavage of said DNA sequence or sequences;(3) DNA topoisomerase activity involved in strand exchange; and (4) DNAligase activity to reseal the cleaved strands of DNA. See Sauer, B.,Current Opinions in Biotechnology 5:521-527 (1994). Conservativesite-specific recombination is distinguished from homologousrecombination and transposition by a high degree of specificity for bothpartners. The strand exchange mechanism involves the cleavage andrejoining of specific DNA sequences in the absence of DNA synthesis(Landy, A. (1989) Ann. Rev. Biochem. 58:913-949).

Subcloning vector: is a cloning vector comprising a circular or linearDNA molecule which includes an appropriate replicon. In the presentinvention, the subcloning vector (segment D in FIG. 1) can also containfunctional and/or regulatory elements that are desired to beincorporated into the final product to act upon or with the cloned DNAInsert (segment A in FIG. 1). The subcloning vector can also contain aSelectable marker (contained in segment C in FIG. 1).

Vector: is a DNA that provides a useful biological or biochemicalproperty to an Insert. Examples include plasmids, phages, and other DNAsequences which are able to replicate or be replicated in vitro or in ahost cell, or to convey a desired DNA segment to a desired locationwithin a host cell. A Vector can have one or more restrictionendonuclease recognition sites at which the DNA sequences can be cut ina determinable fashion without loss of an essential biological functionof the vector, and into which a DNA fragment can be spliced in order tobring about its replication and cloning. Vectors can further provideprimer sites, e.g., for PCR, transcriptional and/or translationalinitiation and/or regulation sites, recombinational signals, replicons,Selectable markers, etc. Clearly, methods of inserting a desired DNAfragment which do not require the use of homologous recombination orrestriction enzymes (such as, but not limited to, UDG cloning of PCRfragments (U.S. Pat. No. 5,334,575, entirely incorporated herein byreference), T:A cloning, and the like) can also be applied to clone afragment of DNA into a cloning vector to be used according to thepresent invention. The cloning vector can further contain a Selectablemarker suitable for use in the identification of cells transformed withthe cloning vector.

Vector Donor: is one of the two parental DNA molecules of the presentinvention which carries the DNA segments encoding the DNA vector whichis to become part of the desired Product. The Vector Donor comprises asubcloning vector D (or it can be called the cloning vector if theInsert Donor does not already contain a cloning vector) and a segment Cflanked by recombination sites (see FIG. 1). Segments C and/or D cancontain elements that contribute to selection for the desired Productdaughter molecule, as described above for selection schemes. Therecombination signals can be the same or different, and can be actedupon by the same or different recombinases. In addition, the VectorDonor can be linear or circular.

Description

One general scheme for an in vitro or in vivo method of the invention isshown in FIG. 1, where the Insert Donor and the Vector Donor can beeither circular or linear DNA, but is shown as circular. Vector D isexchanged for the original cloning vector A. It is desirable to selectfor the daughter vector containing elements A and D and against othermolecules, including one or more Cointegrate(s). The square and circleare different sets of recombination sites (e.g., lox sites or attsites). Segment A or D can contain at least one Selection Marker,expression signals, origins of replication, or specialized functions fordetecting, selecting, expressing, mapping or sequencing DNA, where D isused in this example.

Examples of desired DNA segments that can be part of Element A or Dinclude, but are not limited to, PCR products, large DNA segments,genomic clones or fragments, cDNA clones, functional elements, etc., andgenes or partial genes, which encode useful nucleic acids or proteins.Moreover, the recombinational cloning of the present invention can beused to make ex vivo and in vivo gene transfer vehicles for proteinexpression and/or gene therapy.

In FIG. 1, the scheme provides the desired Product as containing vectorsD and A, as follows. The Insert Donor (containing A and B) is firstrecombined at the square recombination sites by recombination proteins,with the Vector Donor (containing C and D), to form a Co-integratehaving each of A-D-C-B. Next, recombination occurs at the circlerecombination sites to form Product DNA (A and D) and Byproduct DNA (Cand B). However, if desired, two or more different Co-integrates can beformed to generate two or more Products.

In one embodiment of the present in vitro or in vivo recombinationalcloning method, a method for selecting at least one desired Product DNAis provided. This can be understood by consideration of the map ofplasmid pEZC726 depicted in FIG. 2. The two exemplary recombinationsites are attP and loxP. On one segment defined by these sites is akanamycin resistance gene whose promoter has been replaced by the tetOPoperator/promoter from transposon Tn10. In the absence of tet repressorprotein, E. coli RNA polymerase transcribes the kanamycin resistancegene from the tetOP. If tet repressor is present, it binds to tetOP andblocks transcription of the kanamycin resistance gene. The other segmentof pEZC726 has the tet repressor gene expressed by a constitutivepromoter. Thus cells transformed by pEZC726 are resistant tochloramphenicol, because of the chloramphenicol acetyl transferase geneon the same segment as tetR, but are sensitive to kanamycin. Therecombination reactions result in separation of the tetR gene from theregulated kanamycin resistance gene. This separation results inkanamycin resistance in cells receiving the desired recombinationProduct.

Two different sets of plasmids were constructed to demonstrate the invitro method. One set, for use with Cre recombinase only (cloning vector602 and subcloning vector 629 (FIG. 3)) contained loxP and loxP 511sites. A second set, for use with Cre and integrase (cloning vector 705and subcloning vector 726 (FIG. 2)) contained loxP and att sites. Theefficiency of production of the desired daughter plasmid was about 60fold higher using both enzymes than using Cre alone. Nineteen of twentyfour colonies from the Cre-only reaction contained the desired product,while thirty eight of thirty eight colonies from the integrase plus Crereaction contained the desired product plasmid.

Other Selection Schemes A variety of selection schemes can be used thatare known in the art as they can suit a particular purpose for which therecombinational cloning is carried out. Depending upon individualpreferences and needs, a number of different types of selection schemescan be used in the recombinational cloning method of the presentinvention. The skilled artisan can take advantage of the availability ofthe many DNA segments or methods for making them and the differentmethods of selection that are routinely used in the art. Such DNAsegments include but are not limited to those which encodes an activitysuch as, but not limited to, production of RNA, peptide, or protein, orproviding a binding site for such RNA, peptide, or protein. Examples ofDNA molecules used in devising a selection scheme are given above, underthe definition of “selection scheme”

Additional examples include but are not limited to:

-   -   (i) Generation of new primer sites for PCR (e.g., juxtaposition        of two DNA sequences that were not previously juxtaposed);    -   (ii) Inclusion of a DNA sequence acted upon by a restriction        endonuclease or other DNA modifying enzyme, chemical, ribozyme,        etc.;    -   (iii) Inclusion of a DNA sequence recognized by a DNA binding        protein, RNA, DNA, chemical, etc.) (e.g., for use as an affinity        tag for selecting for or excluding from a population) (Davis,        Nucl. Acids Res. 24:702-706 (1996); J. Virol. 69: 8027-8034        (1995));    -   (iv) In vitro selection of RNA ligands for the ribosomal L22        protein associated with Epstein-Barr virus-expressed RNA by        using randomized and cDNA-derived RNA libraries;    -   (vi) The positioning of functional elements whose activity        requires a specific orientation or juxtaposition (e.g., (a) a        recombination site which reacts poorly in trans, but when placed        in cis, in the presence of the appropriate proteins, results in        recombination that destroys certain populations of molecules;        (e.g., reconstitution of a promoter sequence that allows in        vitro RNA synthesis). The RNA can be used directly, or can be        reverse transcribed to obtain the desired DNA construct;    -   (vii) Selection of the desired product by size (e.g.,        fractionation) or other physical property of the molecule(s);        and    -   (viii) Inclusion of a DNA sequence required for a specific        modification (e.g., methylation) that allows its identification.

After formation of the Product and Byproduct in the method of thepresent invention, the selection step can be carried out either in vitroor in vivo depending upon the particular selection scheme which has beenoptionally devised in the particular recombinational cloning procedure.

For example, an in vitro method of selection can be devised for theInsert Donor and Vector Donor DNA molecules. Such scheme can involveengineering a rare restriction site in the starting circular vectors insuch a way that after the recombination events the rare cutting sitesend up in the Byproduct. Hence, when the restriction enzyme which bindsand cuts at the rare restriction site is added to the reaction mixturein vitro, all of the DNA molecules carrying the rare cutting site, i.e.,the starting DNA molecules, the Cointegrate, and the Byproduct, will becut and rendered nonreplicable in the intended host cell. For example,cutting sites in segments B and C (see FIG. 1) can be used to selectagainst all molecules except the Product. Alternatively, only a cuttingsite in C is needed if one is able to select for segment D, e.g., by adrug resistance gene not found on B.

Similarly, an in vitro selection method can be devised when dealing withlinear DNA molecules. DNA sequences complementary to a PCR primersequence can be so engineered that they are transferred, through therecombinational cloning method, only to the Product molecule. After thereactions are completed, the appropriate primers are added to thereaction solution and the sample is subjected to PCR. Hence, all or partof the Product molecule is amplified.

Other in vivo selection schemes can be used with a variety of E. colicell lines. One is to put a repressor gene on one segment of thesubcloning plasmid, and a drug marker controlled by that repressor onthe other segment of the same plasmid. Another is to put a killer geneon segment C of the subcloning plasmid (FIG. 1). Of course a way mustexist for growing such a plasmid, i.e., there must exist circumstancesunder which the killer gene will not kill. There are a number of thesegenes known which require particular strains of E. coli. One such schemeis to use the restriction enzyme DpnI, which will not cleave unless itsrecognition sequence GATC is methylated. Many popular common E. colistrains methylate GATC sequences, but there are mutants in which clonedDpnI can be expressed without harm.

Of course analogous selection schemes can be devised for other hostorganisms. For example, the tet repressor/operator of Tn10 has beenadapted to control gene expression in eukaryotes (Gossen, M., andBujard, H., Proc. Natl. Acad. Sci. USA 89:5547-5551 (1992)). Thus thesame control of drug resistance by the tet repressor exemplified hereincan be applied to select for Product in eukaryotic cells.

Recombination Proteins

In the present invention, the exchange of DNA segments is achieved bythe use of recombination proteins, including recombinases and associatedco-factors and proteins. Various recombination proteins are described inthe art. Examples of such recombinases include:

Cre: A protein from bacteriophage PI (Abremski and Hoess, J. Biol. Chem.259(3):1509-1514 (1984)) catalyzes the exchange (i.e., causesrecombination) between 34 bp DNA sequences called loxP (locus ofcrossover) sites (See Hoess et al., Nucl. Acids Res. 14(5):2287 (1986)).Cre is available commercially (Novagen, Catalog No. 69247-1).Recombination mediated by Cre is freely reversible. From thermodynamicconsiderations it is not surprising that Cre-mediated integration(recombination between two molecules to form one molecule) is much lessefficient than Cre-mediated excision (recombination between two loxPsites in the same molecule to form two daughter molecules). Cre works insimple buffers with either magnesium or spermidine as a cofactor, as iswell known in the art. The DNA substrates can be either linear orsupercoiled. A number of mutant loxP sites have been described (Hoess etal., supra). One of these, loxP 511, recombines with another loxP 511site, but will not recombine with a loxP site.

Integrase: A protein from bacteriophage lambda that mediates theintegration of the lambda genome into the E. coli chromosome. Thebacteriophage λ Int recombinational proteins promote irreversiblerecombination between its substrate att sites as part of the formationor induction of a lysogenic state. Reversibility of the recombinationreactions results from two independent pathways for integrative andexcisive recombination. Each pathway uses a unique, but overlapping, setof the 15 protein binding sites that comprise att site DNAs. Cooperativeand competitive interactions involving four proteins (Int, Xis, IHF andFIS) determine the direction of recombination.

Integrative recombination involves the Int and IHF proteins and sitesattP (240 bp) and attB (25 bp). Recombination results in the formationof two new sites: attL and attR. Excisive recombination requires Int,IHF, and Xis, and sites attL and attR to generate attP and attB. Undercertain conditions, FIS stimulates excisive recombination. In additionto these normal reactions, it should be appreciated that attP and attB,when placed on the same molecule, can promote excisive recombination togenerate two excision products, one with attL and one with attR.Similarly, intermolecular recombination between molecules containingattL and attR, in the presence of Int, IHF and Xis, can result inintegrative recombination and the generation attP and attB. Hence, byflanking DNA segments with appropriate combinations of engineered attsites, in the presence of the appropriate recombination proteins, onecan direct excisive or integrative recombination, as reverse reactionsof each other.

Each of the att sites contains a 15 bp core sequence; individualsequence elements of functional significance lie within, outside, andacross the boundaries of this common core (Landy, A., Ann. Rev. Biochem.58:913 (1989)). Efficient recombination between the various att sitesrequires that the sequence of the central common region be identicalbetween the recombining partners, however, the exact sequence is nowfound to be modifiable. Consequently, derivatives of the att site withchanges within the core are now discovered to recombine as least asefficiently as the native core sequences.

Integrase acts to recombine the attP site on bacteriophage lambda (about240 bp) with the attB site on the E. coli genome (about 25 bp)(Weisberg, R. A. and Landy, A. in Lambda II, p. 211 (1983), Cold SpringHarbor Laboratory)), to produce the integrated lambda genome flanked byattL (about 100 bp) and attR (about 160 bp) sites. In the absence of Xis(see below), this reaction is essentially irreversible. The integrationreaction mediated by integrase and IHF works in vitro, with simplebuffer containing spermidine. Integrase can be obtained as described byNash, H. A., Methods of Enzymology 100:210-216 (1983). IHF can beobtained as described by Filutowicz, M., et al., Gene 147:149-150(1994).

In the presence of the λ protein Xis (excise) integrase catalyzes thereaction of attR and attL to form attP and attB, i.e., it promotes thereverse of the reaction described above. This reaction can also beapplied in the present invention.

Other Recombination Systems. Numerous recombination systems from variousorganisms can also be used, based on the teaching and guidance providedherein. See, e.g., Hoess et al., Nucleic Acids Research 14(6):2287(1986); Abremski et al., J. Biol. Chem. 261(1):391 (1986); Campbell, J.Bacteriol 174(23):7495 (1992); Qian et al., J. Biol. Chem. 267(11):7794(1992); Araki et al., J. Mol. Biol. 225(1):25 (1992)). Many of thesebelong to the integrase family of recombinases (Argos et al. EMBO J.5:433-440 (1986)). Perhaps the best studied of these are thelntegrase/att system from bacteriophage λ (Landy, A. (1993) CurrentOpinions in Genetics and Devel. 3:699-707), the Cre/loxP system frombacteriophage P1 (Hoess and Abremski (1990) In Nucleic Acids andMolecular Biology, vol. 4. Eds.: Eckstein and Lilley, Berlin-Heidelberg:Springer-Verlag; pp. 90-109), and the FLP/FRT system from theSaccharomyces cerevisiae 2μ circle plasmid (Broach et al. Cell29:227-234 (1982)).

Members of a second family of site-specific recombinases, the resolvasefamily (e.g., γδ, Tn3 resolvase, Hin, Gin, and Cin) are also known.Members of this highly related family of recombinases are typicallyconstrained to intramolecular reactions (e.g., inversions and excisions)and can require host-encoded factors. Mutants have been isolated thatrelieve some of the requirements for host factors (Maeser and Kahnmann(1991) Mol. Gen. Genet. 230:170-176), as well as some of the constraintsof intramolecular recombination.

Other site-specific recombinases similar to λ Int and similar to P1 Crecan be substituted for Int and Cre. Such recombinases are known. In manycases the purification of such other recombinases has been described inthe art. In cases when they are not known, cell extracts can be used orthe enzymes can be partially purified using procedures described for Creand Int.

While Cre and Int are described in detail for reasons of example, manyrelated recombinase systems exist and their application to the describedinvention is also provided according to the present invention. Theintegrase family of site-specific recombinases can be used to providealternative recombination proteins and recombination sites for thepresent invention, as site-specific recombination proteins encoded bybacteriophage lambda, phi 80, P22, P2, 186, P4 and P1. This group ofproteins exhibits an unexpectedly large diversity of sequences. Despitethis diversity, all of the recombinases can be aligned in theirC-terminal halves.

A 40-residue region near the C terminus is particularly well conservedin all the proteins and is homologous to a region near the C terminus ofthe yeast 2 mu plasmid Flp protein. Three positions are perfectlyconserved within this family: histidine, arginine and tyrosine are foundat respective alignment positions 396, 399 and 433 within thewell-conserved C-terminal region. These residues contribute to theactive site of this family of recombinases, and suggest thattyrosine-433 forms a transient covalent linkage to DNA during strandcleavage and rejoining. See, e.g., Argos, P. et al., EMBO J. 5:433-40(1986).

Alternatively, IS231 and other Bacillus thuringiensis transposableelements could be used as recombination proteins and recombinationsites. Bacillus thuringiensis is an entomopathogenic bacterium whosetoxicity is due to the presence in the sporangia of delta-endotoxincrystals active against agricultural pests and vectors of human andanimal diseases. Most of the genes coding for these toxin proteins areplasmid-borne and are generally structurally associated with insertionsequences (IS231, IS232, IS240, ISBT1 and ISBT2) and transposons (Tn4430and Tn5401). Several of these mobile elements have been shown to beactive and participate in the crystal gene mobility, therebycontributing to the variation of bacterial toxicity.

Structural analysis of the iso-IS231 elements indicates that they arerelated to IS1151 from Clostridium perfringens and distantly related toIS4 and IS186 from Escherichia coli. Like the other IS4 family members,they contain a conserved transposase-integrase motif found in other ISfamilies and retroviruses.

Moreover, functional data gathered from IS231A in Escherichia coliindicate a non-replicative mode of transposition, with a preference forspecific targets. Similar results were also obtained in Bacillussubtilis and B. thuringiensis. See, e.g., Mahillon, J. et al., Genetica93:13-26 (1994); Campbell, J. Bacteriol. 7495-7499 (1992).

The amount of recombinase which is added to drive the recombinationreaction can be determined by using known assays. Specifically,titration assay is used to determine the appropriate amount of apurified recombinase enzyme, or the appropriate amount of an extract.

Engineered Recombination Sites. The above recombinases and correspondingrecombinase sites are suitable for use in recombination cloningaccording to the present invention. However, wild-type recombinationsites contain sequences that reduce the efficiency or specificity ofrecombination reactions as applied in methods of the present invention.For example, multiple stop codons in attB, attR, attP, attL and loxprecombination sites occur in multiple reading frames on both strands, sorecombination efficiencies are reducted, e.g., where the coding sequencemust cross the recombination sites, (only one reading frame is availableon each strand of loxP and attB sites) or impossible (in attP, attR orattL).

Accordingly, the present invention also provides engineeredrecombination sites that overcome these problems. For example, att sitescan be engineered to have one or multiple mutations to enhancespecificity or efficiency of the recombination reaction and theproperties of Product DNAs (e.g., att1, att2, and att3 sites); todecrease reverse reaction (e.g., removing P1 and H1 from attB). Thetesting of these mutants determines which mutants yield sufficientrecombinational activity to be suitable for recombination subcloningaccording to the present invention.

Mutations can therefore be introduced into recombination sites forenhancing site specific recombination. Such mutations include, but arenot limited to: recombination sites without translation stop codons thatallow fusion proteins to be encoded; recombination sites recognized bythe same proteins but differing in base sequence such that they reactlargely or exclusively with their homologous partners allow multiplereactions to be contemplated. Which particular reactions take place canbe specified by which particular partners are present in the reactionmixture. For example, a tripartite protein fusion could be accomplishedwith parental plasmids containing recombination sites attR1 and attR2;attL1 and attL3; and/or attR3 and attL2.

There are well known procedures for introducing specific mutations intonucleic acid sequences. A number of these are described in Ausubel, F.M. et al., Current Protocols in Molecular Biology, Wiley Interscience,New York (1989-1996). Mutations can be designed into oligonucleotides,which can be used to modify existing cloned sequences, or inamplification reactions. Random mutagenesis can also be employed ifappropriate selection methods are available to isolate the desiredmutant DNA or RNA. The presence of the desired mutations can beconfirmed by sequencing the nucleic acid by well known methods.

The following non-limiting methods can be used to engineer a core regionof a given recombination site to provide mutated sites suitable for usein the present invention:

-   -   1. By recombination of two parental DNA sequences by        site-specific (e.g. attL and attR to give attB) or other (e.g.        homologous) recombination mechanisms. The DNA parental DNA        segments containing one or more base alterations resulting in        the final core sequence;    -   2. By mutation or mutagenesis (site-specific, PCR, random,        spontaneous, etc) directly of the desired core sequence;    -   3. By mutagenesis (site-specific, PCR, random, spontanteous,        etc) of parental DNA sequences, which are recombined to generate        a desired core sequence; and    -   4. By reverse transcription of an RNA encoding the desired core        sequence.

The functionality of the mutant recombination sites can be demonstratedin ways that depend on the particular characteristic that is desired.For example, the lack of translation stop codons in a recombination sitecan be demonstrated by expressing the appropriate fusion proteins.Specificity of recombination between homologous partners can bedemonstrated by introducing the appropriate molecules into in vitroreactions, and assaying for recombination products as described hereinor known in the art. Other desired mutations in recombination sitesmight include the presence or absence of restriction sites, translationor transcription start signals, protein binding sites, and other knownfunctionalities of nucleic acid base sequences. Genetic selectionschemes for particular functional attributes in the recombination sitescan be used according to known method steps. For example, themodification of sites to provide (from a pair of sites that do notinteract) partners that do interact could be achieved by requiringdeletion, via recombination between the sites, of a DNA sequenceencoding a toxic substance. Similarly, selection for sites that removetranslation stop sequences, the presence or absence of protein bindingsites, etc., can be easily devised by those skilled in the art.

Accordingly, the present invention provides a nucleic acid molecule,comprising at least one DNA segment having at least two engineeredrecombination sites flanking a Selectable marker and/or a desired DNAsegment, wherein at least one of said recombination sites comprises acore region having at least one engineered mutation that enhancesrecombination in vitro in the formation of a Cointegrate DNA or aProduct DNA.

The nucleic acid molecule can have at least one mutation that confers atleast one enhancement of said recombination, said enhancement selectedfrom the group consisting of substantially (i) favoring excisiveintegration; (ii) favoring excisive recombination; (ii) relieving therequirement for host factors; (iii) increasing the efficiency of saidCointegrate DNA or Product DNA formation; and (iv) increasing thespecificity of said Cointegrate DNA or Product DNA formation.

The nucleic acid molecule preferably comprises at least onerecombination site derived from attB, attP, attL or attR. Morepreferably the att site is selected from att1, att2, or att3, asdescribed herein.

In a preferred embodiment, the core region comprises a DNA sequenceselected from the group consisting of: (a) RKYCWGCTTTYKTRTACNAASTSGB(SEQ ID NO: 1) (m-att); (b) AGCCWGCTTTYKTRTACNAACTSGB (SEQ ID NO: 2)(m-attB); (c) GTTCAGCTTTCKTRTACNAACTSGB (SEQ ID NO: 3) (m-attR); (d)AGCCWGCTTTCKTRTACNAAGTSGB (SEQ ID NO: 4) (m-attL); (e)GTTCAGCTTTYKTRTACNAAGTSGB (SEQ ID NO: 5) (m-attP1);or a corresponding or complementary DNA or RNA sequence, wherein R=A orG; K=G or T/U; Y=C or T/U; W=A or T/U; N=A or C or G or T/U; S=Cor G;and B=C or G or T/U, as presented in 37 C.F.R. §1.822, which is entirelyincorporated herein by reference, wherein the core region does notcontain a stop codon in one or more reading frames.

The core region also preferably comprises a DNA sequence selected fromthe group consisting of: (a) AGCCTGCTTTTTTGTACAAACTTGT (SEQ ID NO: 6)(attB1); (b) AGCCTGCTTTCTTGTACAAACTTGT (SEQ ID NO: 7) (attB2); (c)ACCCAGCTTTCTTGTACAAACTTGT (SEQ ID NO: 8) (attB3); (d)GTTCAGCTTTTTTGTACAAACTTGT (SEQ ID NO: 9) (attR1); (e)GTTCAGCTTTCTTGTACAAACTTGT (SEQ ID NO: 10) (attR2); (f)GTTCAGCTTTCTTGTACAAAGTTGG (SEQ ID NO: 11) (attR3); (g)AGCCTGCTTTTTTGTACAAAGTTGG (SEQ ID NO: 12) (attL1); (h)AGCCTGCTTTCTTGTACAAAGTTGG (SEQ ID NO: 13) (attL2); (i)ACCCAGCTTTCTTGTACAAAGTTGG (SEQ ID NO: 14) (attL3); (j)GTTCAGCTTTTTTGTACAAAGTTGG (SEQ ID NO: 15) (attP1); (k)GTTCAGCTTTCTTGTACAAAGTTGG (SEQ ID NO: 16) (attP2, P3);or a corresponding or complementary DNA or RNA sequence.

The present invention thus also provides a method for making a nucleicacid molecule, comprising providing a nucleic acid molecule having atleast one engineered recombination site comprising at least one DNAsequence having at least 80-99% homology (or any range or value therein)to at least one of SEQ ID NOS:1-6, or any suitable recombination site,or which hybridizes under stringent conditions thereto, as known in theart.

Clearly, there are various types and permutations of such well-known invitro and in vivo selection methods, each of which are not describedherein for the sake of brevity. However, such variations andpermutations are contemplated and considered to be the differentembodiments of the present invention.

It is important to note that as a result of the preferred embodimentbeing in vitro recombination reactions, non-biological molecules such asPCR products can be manipulated via the present recombinational cloningmethod. In one example, it is possible to clone linear molecules intocircular vectors.

There are a number of applications for the present invention. These usesinclude, but are not limited to, changing vectors, apposing promoterswith genes, constructing genes for fusion proteins, changing copynumber, changing replicons, cloning into phages, and cloning, e.g., PCRproducts (with an attB site at one end and a loxP site at the otherend), genomic DNAs, and cDNAs.

The following examples are intended to further illustrate certainpreferred embodiments of the invention and are not intended to belimiting in nature.

EXAMPLES

The present recombinational cloning method accomplishes the exchange ofnucleic acid segments to render something useful to the user, such as achange of cloning vectors. These segments must be flanked on both sidesby recombination signals that are in the proper orientation with respectto one another. In the examples below the two parental nucleic acidmolecules (e.g., plasmids) are called the Insert Donor and the VectorDonor. The Insert Donor contains a segment that will become joined to anew vector contributed by the Vector Donor. The recombinationintermediate(s) that contain(s) both starting molecules is called theCointegrate(s). The second recombination event produces two daughtermolecules, called the Product (the desired new clone) and the Byproduct.

Buffers

Various known buffers can be used in the reactions of the presentinvention. For restriction enzymes, it is advisable to use the buffersrecommended by the manufacturer. Alternative buffers can be readilyfound in the literature or can be devised by those of ordinary skill inthe art.

Examples 1-3. One exemplary buffer for lambda integrase is comprised of50 mM Tris-HCl, at pH 7.5-7.8, 70 mM KCl, 5 mM spermidine, 0.5 mM EDTA,and 0.25 mg/ml bovine serum albumin, and optionally, 10% glycerol.

One preferred buffer for P1 Cre recombinase is comprised of 50 mMTris-HCl at pH 7.5, 33 mM NaCl, 5 mM spermidine, and 0.5 mg/ml bovineserum albumin.

The buffer for other site-specific recombinases which are similar tolambda Int and P1 Cre are either known in the art or can be determinedempirically by the skilled artisans, particularly in light of theabove-described buffers.

Example 1 Recombinational Cloning Using Cre and Cre & Int

Two pairs of plasmids were constructed to do the in vitrorecombinational cloning method in two different ways. One pair, pEZC705and pEZC726 (FIG. 2A), was constructed with loxP and att sites, to beused with Cre and λ integrase. The other pair, pEZC602 and pEZC629 (FIG.3A), contained the loxP (wild type) site for Cre, and a second mutantlox site, loxP 511, which differs from loxP in one base (out of 34total). The minimum requirement for recombinational cloning of thepresent invention is two recombination sites in each plasmid, in generalX and Y, and X′ and Y′. Recombinational cloning takes place if either orboth types of site can recombine to form a Cointegrate (e.g. X and X′),and if either or both (but necessarily a site different from the typeforming the Cointegrate) can recombine to excise the Product andByproduct plasmids from the Cointegrate (e.g. Y and Y′). It is importantthat the recombination sites on the same plasmid do not recombine. Itwas found that the present recombinational cloning could be done withCre alone.

Cre-Only

Two plasmids were constructed to demonstrate this conception (see FIG.3A). pEZC629 was the Vector Donor plasmid. It contained a constitutivedrug marker (chloramphenicol resistance), an origin of replication, loxPand loxP 511 sites, a conditional drug marker (kanamycin resistancewhose expression is controlled by the operator/promoter of thetetracycline resistance operon of transposon Tn10), and a constitutivelyexpressed gene for the tet repressor protein, tetR. E. coli cellscontaining pEZC629 were resistant to chloramphenicol at 30 μg/ml, butsensitive to kanamycin at 100 μg/ml. pEZC602 was the Insert Donorplasmid, which contained a different drug marker (ampicillinresistance), an origin, and loxP and loxP 511 sites flanking a multiplecloning site.

This experiment was comprised of two parts as follows:

Part I: About 75 ng each of pEZC602 and pEZC629 were mixed in a totalvolume of 30 μl of Cre buffer (50 mM Tris-HCl pH 7.5, 33 mM NaCl, 5 mMspermidine-HCl, 500 μg/ml bovine serum albumin). Two 10 μl aliquots weretransferred to new tubes. One tube received 0.5 μl of Cre protein(approx. 4 units per μl; partially purified according to Abremski andHoess, J. Biol. Chem. 259:1509 (1984)). Both tubes were incubated at 37°C. for 30 minutes, then 70° C. for 10 minutes. Aliquots of each reactionwere diluted and transformed into DH5α. Following expression, aliquotswere plated on 30 μg/ml chloramphenicol; 100 μg/ml ampicillin plus 200μg/ml methicillin; or 100 μg/ml kanamycin. Results: See Table 1. Thereaction without Cre gave 1.11×10⁶ ampicillin resistant colonies (fromthe Insert Donor plasmid pEZC602); 7.8×10⁵ chloramphenicol resistantcolonies (from the Vector Donor plasmid pEZC629); and 140 kanamycinresistant colonies (background). The reaction with added Cre gave7.5×10⁵ ampicillin resistant colonies (from the Insert Donor plasmidpEZC602); 6.1×10⁵ chloramphenicol resistant colonies (from the VectorDonor plasmid pEZC629); and 760 kanamycin resistant colonies (mixture ofbackground colonies and colonies from the recombinational cloningProduct plasmid). Analysis: Because the number of colonies on thekanamycin plates was much higher in the presence of Cre, many or most ofthem were predicted to contain the desired Product plasmid. TABLE 1Chlo- Enzyme Ampicillin ramphenicol Kanamycin Efficiency None 1.1 × 10⁶7.8 × 10⁵ 140 140/7.8 × 10⁵ = 0.02% Cre 7.5 × 10⁵ 6.1 × 10⁵ 760 760/6.1× 10⁵ = 0.12%

Part II: Twenty four colonies from the “+Cre” kanamycin plates werepicked and inoculated into medium containing 100 μg/ml kanamycin.Minipreps were done, and the miniprep DNAs, uncut or cut with SmaI orHindIII, were electrophoresed. Results: 19 of the 24 minipreps showedsupercoiled plasmid of the size predicted for the Product plasmid. All19 showed the predicted SmaI and HindIII restriction fragments.Analysis: The Cre only scheme was demonstrated. Specifically, it wasdetermined to have yielded about 70% (19 of 24) Product clones. Theefficiency was about 0.1% (760 kanamycin resistant clones resulted from6.1×10⁵ chloramphenicol resistant colonies).

Cre Plus Integrase

The plasmids used to demonstrate this method are exactly analogous tothose used above, except that pEZC726, the Vector Donor plasmid,contained an attP site in place of loxP 511, and pEZC705, the InsertDonor plasmid, contained an attB site in place of loxP 511 (FIG. 2A).

This experiment was comprised of three parts as follows:

Part I: About 500 ng of pEZC705 (the Insert Donor plasmid) was cut withScaI, which linearized the plasmid within the ampicillin resistancegene. (This was done because the λ integrase reaction has beenhistorically done with the attB plasmid in a linear state (H. Nash,personal communication). However, it was found later that the integrasereaction proceeds well with both plasmids supercoiled.) Then, the linearplasmid was ethanol precipitated and dissolved in 20 μl of λ integrasebuffer (50 mM Tris-HCl, about pH 7.8, 70 mM KCl, 5 mM spermidine-HCl,0.5 mM EDTA, 250 μg/ml bovine serum albumin). Also, about 500 ng of theVector Donor plasmid pEZC726 was ethanol precipitated and dissolved in20 μl λ integrase buffer. Just before use, λ integrase (2 μl, 393 μg/ml)was thawed and diluted by adding 18 μl cold λ integrase buffer. One μlIHF (integration host factor, 2.4 mg/ml, an accessory protein) wasdiluted into 150 μl cold λ integrase buffer. Aliquots (2 μl) of each DNAwere mixed with λ integrase buffer, with or without 1 μl each λintegrase and IHF, in a total of 10 μl. The mixture was incubated at 25°C. for 45 minutes, then at 70° C. for 10 minutes. Half of each reactionwas applied to an agarose gel. Results: In the presence of integrase andIHF, about 5% of the total DNA was converted to a linear Cointegrateform. Analysis: Activity of integrase and IHF was confirmed.

Part II: Three microliters of each reaction (i.e., with or withoutintegrase and IHF) were diluted into 27 μl of Cre buffer (above), theneach reaction was split into two 10 μl aliquots (four altogether). Totwo of these reactions, 0.5 μl of Cre protein (above) were added, andall reactions were incubated at 37° C. for 30 minutes, then at 70° C.for 10 minutes. TE buffer (90 μl; TE: 10 mM Tris-HCl, pH 7.5, 1 mM EDTA)was added to each reaction, and 1 μl each was transformed into E. coliDH5α. The transformation mixtures were plated on 100 μg/ml ampicillinplus 200 μg/ml methicillin; 30 μg/ml chloramphenicol; or 100 μg/mlkanamycin. Results: See Table 2. TABLE 2 Chloram- Kana- EnzymeAmpicillin phenicol mycin Efficiency None 990 20000 4 4/2 × 10⁴ = 0.02%Cre only 280 3640 0 0 Integrase* 1040 27000 9 9/2.7 × 10⁴ = 0.03% onlyIntegrase* + Cre 110 1110 76 76/1.1 × 10³ = 6.9%*Integrase reactions also contained IHF.

Analysis: The Cre protein impaired transformation. When adjusted forthis effect, the number of kanamycin resistant colonies, compared to thecontrol reactions, increased more than 100 fold when both Cre andIntegrase were used. This suggests a specificity of greater than 99%.

Part III: 38 colonies were picked from the Integrase plus Cre plates,miniprep DNAs were made and cut with HindIII to give diagnostic mappinginformation. Result: All 38 had precisely the expected fragment sizes.Analysis: The Cre plus λ integrase method was observed to have muchhigher specificity than Cre-alone. Conclusion: The Cre plus λ integrasemethod was demonstrated. Efficiency and specificity were much higherthan for Cre only.

Example 2 Using in vitro Recombinational Cloning to Subclone theChloramphenicol Acetyl Transferase Gene into a Vector for Expression inEukaryotic Cells (FIG. 4A)

An Insert Donor plasmid, pEZC843, was constructed, comprising thechloramphenicol acetyl transferase gene of E. coli, cloned between loxPand attB sites such that the loxP site was positioned at the 5′-end ofthe gene (FIG. 4B). A Vector Donor plasmid, pEZC1003, was constructed,which contained the cytomegalovirus eukaryotic promoter apposed to aloxP site (FIG. 4C). One microliter aliquots of each supercoiled plasmid(about 50 ng crude miniprep DNA) were combined in a ten microliterreaction containing equal parts of lambda integrase buffer (50 mMTris-HCl, pH 7.8, 70 mM KCl, 5 mM spermidine, 0.5 mM EDTA, 0.25 mg/mlbovine serum albumin) and Cre recombinase buffer (50 mM Tris-HCl, pH7.5, 33 mM NaCl, 5 mM spermidine, 0.5 mg/ml bovine serum albumin), twounits of Cre recombinase, 16 ng integration host factor, and 32 nglambda integrase. After incubation at 30° C. for 30 minutes and 75° C.for 10 minutes, one microliter was transformed into competent E. colistrain DH5α (Life Technologies, Inc.). Aliquots of transformations werespread on agar plates containing 200 μg/ml kanamycin and incubated at37° C. overnight. An otherwise identical control reaction contained theVector Donor plasmid only. The plate receiving 10% of the controlreaction transformation gave one colony; the plate receiving 10% of therecombinational cloning reaction gave 144 colonies. These numberssuggested that greater than 99% of the recombinational cloning coloniescontained the desired product plasmid. Miniprep DNA made from sixrecombinational cloning colonies gave the predicted size plasmid (5026base pairs), CMVProd. Restriction digestion with NcoI gave the fragmentspredicted for the chloramphenicol acetyl transferase cloned downstreamof the CMV promoter for all six plasmids.

Example 3 Subcloned DNA Segments Flanked by attB Sites Without StopCodons

Part I: Background

The above examples are suitable for transcriptional fusions, in whichtranscription crosses recombination sites. However, both attR and loxPsites contain multiple stop codons on both strands, so translationalfusions can be difficult, where the coding sequence must cross therecombination sites, (only one reading frame is available on each strandof loxP sites) or impossible (in attR or attL).

A principal reason for subcloning is to fuse protein domains. Forexample, fusion of the glutathione S-transferase (GST) domain to aprotein of interest allows the fusion protein to be purified by affinitychromatography on glutathione agarose (Pharmacia, Inc., 1995 catalog).If the protein of interest is fused to runs of consecutive histidines(for example His6), the fusion protein can be purified by affinitychromatography on chelating resins containing metal ions (Qiagen, Inc.).It is often desirable to compare amino terminal and carboxy terminalfusions for activity, solubility, stability, and the like.

The attB sites of the bacteriophage X integration system were examinedas an alternative to loxP sites, because they are small (25 bp) and havesome sequence flexibility (Nash, H. A. et al., Proc. Natl. Acad. Sci.USA 84:4049-4053 (1987). It was not previously suggested that multiplemutations to remove all stop codes would result in useful recombinationsites for recombinational subcloning.

Using standard nomenclature for site specific recombination in lambdabacteriophage (Weisber, in Lambda III, Hendrix, et al., eds., ColdSpring Harbor Laboratory, Cold Spring Harbor, N.Y. (1989)), thenucleotide regions that participate in the recombination reaction in anE. coli host cell are represented as follows:

where: O represents the 15 bp core DNA sequence found in both the phageand E. coli genomes; B and B′ represent approximately 5 bases adjacentto the core in the E. coli genome; and P1, H1, P2, X, H2, C, C′, H′,P′1, P′2, and P′3 represent known DNA sequences encoding protein bindingdomains in the bacteriophage λ genome.

The reaction is reversible in the presence of the protein Xis(excisionase); recombination between attL and attR precisely excise theλ genome from its integrated state, regenerating the circular λ genomecontaining attP and the linear E. coli genome containing attB.

Part II: Construction and Testing of Plasmids Containing Mutant attSites

Mutant attL and attR sites were constructed. Importantly, Landy et al.(Ann. Rev. Biochem. 58:913 (1989)) observed that deletion of the P1 andH1 domains of attP facilitated the excision reaction and eliminated theintegration reaction, thereby making the excision reaction irreversible.Therefore, as mutations were introduced in attR, the P1 and H1 domainswere also deleted. attR sites in the present example lack the P1 and H1regions and have the NdeI site removed (base 27630 changed from C to G),and contain sequences corresponding to bacteriophage λ coordinates27619-27738 (GenBank release 92.0, bg:LAMCG, “Complete Sequence ofBacteriophage Lambda”).

The sequence of attB produced by recombination of wild type attL andattR sites is: (SEQ. ID NO 31) B            O            B′ attBwt: 5′AGCCT GCTTTTTTATAC TAA CTTGA 3′ (SEQ. ID NO: 32) 3′ TCGGA CGAAAA AAT ATGAT T GAACT 5′The stop codons are italicized and underlined. Note that sequences ofattL, attR, and attP can be derived from the attB sequence and theboundaries of bacteriophage λ contained within attL and attR(coordinates 27619 to 27818).

When mutant attR1 and attL1 sites were recombined the sequence attB1 wasproduced (mutations in bold, large font): (SEQ ID NO: 6)B            O            B′ attB1: 5′ AGCCT GCTTTTTT

TAC

AA CTTG

3′ (SEQ. ID NO: 33) 3′ TCGGA CGAAAAAA

ATG

TT GAAC

5′Note that the four stop codons are gone.

When an additional mutation was introduced in the attR1 and attL1sequences (bold), attR2 and attL2 sites resulted. Recombination of attR2and attL2 produced the attB2 site: (SEQ. ID NO: 7)B            O            B′ attB2: 5′ AGCCT GCTTT

TTGTACAAA CTTGT 3′ 3′ TCGGA CGAAA

AACATGTTT GAACA 5′ (SEQ. ID NO: 34)

The recombination activities of the above attL and attR sites wereassayed as follows. The attB site of plasmid pEZC705 (FIG. 2B) wasreplaced with attLwt, attL1, or attL2. The attP site of plasmid pEZC726(FIG. 2C) was replaced with attRwt (lacking regions P1 and H1), attR1,or attR2. Thus, the resulting plasmids could recombine via their loxPsites, mediated by Cre, and via their attR and attL sites, mediated byInt, Xis, and IHF. Pairs of plasmids were mixed and reacted with Cre,Int, Xis, and IHF, transformed into E. coli competent cells, and platedon agar containing kanamycin. The results are presented in Table 3:TABLE 3 # of kanamycin resistant Vector donor att site Gene donor attsite colonies* attRwt (pEZC1301) None  1 (background) ″ attLwt(pEZC1313) 147 ″ attL1 (pEZC1317)  47 ″ attL2 (pEZC1321)  0 attR1(pEZC1305) None  1 (background) ″ attLwt (pEZC1313)  4 ″ attL1(pEZC1317) 128 ″ attL2 (pEZC1321)  0 attR2 (pEZC1309) None  9(background) ″ attLwt (pEZC1313)  0 ″ attL2 (pEZC1317)  0 ″ attL2(pEZC1321) 209(*1% of each transformation was spread on a kanamycin plate.)

The above data show that whereas the wild type att and att1 sitesrecombine to a small extent, the att1 and att2 sites do not recombinedetectably with each other.

Part III. Recombination was demonstrated when the core region of bothattB sites flanking the DNA segment of interest did not contain stopcodons. The physical state of the participating plasmids was discoveredto influence recombination efficiency.

The appropriate att sites were moved into pEZC705 and pEZC726 to makethe plasmids pEZC1405 (FIG. 5G) (attR1 and attR2) and pEZC1502 (FIG. 5H)(attL1 and attL2). The desired DNA segment in this experiment was a copyof the chloramphenicol resistance gene cloned between the two attL sitesof pEZC1502. Pairs of plasmids were recombined in vitro using Int, Xis,and IHF (no Cre because no loxP sites were present). The yield ofdesired kanamycin resistant colonies was determined when both parentalplasmids were circular, or when one plasmid was circular and the otherlinear as presented in Table 4: TABLE 4 Kanamycin resistant Vectordonor¹ Gene donor¹ colonies² Circular pEZC1405 None 30 Circular pEZC1405Circular pEZC1502 2680 Linear pEZC1405 None 90 Linear pEZC1405 CircularpEZC1502 172000 Circular pEZC1405 Linear pEZC1502 73000¹DNAs were purified with Qiagen columns, concentrations determined byA260, and linearized with Xba I (pEZC1405) or AlwN I (pEZC1502). Eachreaction contained 100 ng of the indicated DNA. All reactions (10 μltotal) contained 3 μl of enzyme# mix (Xis, Int, and IHF). After incubation (45 minutes at 25°, 10minutes at 65°), one μl was used to transform E. coli DH5α cells.²Number of colonies expected if the entire transformation reaction (1ml) had been plated. Either 100 μl or 1 μl of the transformations wereactually plated.

Analysis: Recombinational cloning using mutant attR and attL sites wasconfirmed. The desired DNA segment is subcloned between attB sites thatdo not contain any stop codons in either strand. The enhanced yield ofProduct DNA (when one parent was linear) was unexpected because ofearlier observations that the excision reaction was more efficient whenboth participating molecules were supercoiled and proteins were limiting(Nunes-Duby et al., Cell 50:779-788 (1987).

Example 4 Demonstration of Recombinational Cloning Without InvertedRepeats

Part I: Rationale

The above Example 3 showed that plasmids containing inverted repeats ofthe appropriate recombination sites (for example, attL1 and attL2 inplasmid pEZC1502) (FIG. 5H) could recombine to give the desired DNAsegment flanked by attB sites without stop codons, also in invertedorientation. A concern was the in vivo and in vitro influence of theinverted repeats. For example, transcription of a desired DNA segmentflanked by attB sites in inverted orientation could yield a singlestranded RNA molecule that might form a hairpin structure, therebyinhibiting translation.

Inverted orientation of similar recombination sites can be avoided byplacing the sites in direct repeat arrangement att sites. If parentalplasmids each have a wild type attL and wild type attR site, in directrepeat the Int, Xis, and IHF proteins will simply remove the DNA segmentflanked by those sites in an intramolecular reaction. However, themutant sites described in the above Example 3 suggested that it might bepossible to inhibit the intramolecular reaction while allowing theintermolecular recombination to proceed as desired.

Part II: Structure of Plasmids Without Inverted Repeats forRecombinational Cloning

The attR2 sequence in plasmid pEZC1405 (FIG. 5G) was replaced withattL2, in the opposite orientation, to make pEZC1603 (FIG. 6A). TheattL2 sequence of pEZC1502 (FIG. 5H) was replaced with attR2, in theopposite orientation, to make pEZC1706 (FIG. 6B). Each of these plasmidscontained mutations in the core region that make intramolecularreactions between att1 and att2 cores very inefficient (see Example 3,above).

Plasmids pEZC1405, pEZC1502, pEZC1603 and pEZC1706 were purified onQiagen columns (Qiagen, Inc.). Aliquots of plasmids pEZC1405 andpEZC1603 were linearized with Xba I. Aliquots of plasmids pEZC1502 andpEZC1706 were linearized with AlwN I. One hundred ng of plasmids weremixed in buffer (equal volumes of 50 mM Tris HCl pH 7.5, 25 mM Tris HClpH 8.0, 70 mM KCl, 5 mM spermidine, 0.5 mM EDTA, 250 μg/ml BSA, 10%glycerol) containing Int (43.5 ng), Xis (4.3 ng) and IHF (8.1 ng) in afinal volume of 10 μl. Reactions were incubated for 45 minutes at 25°C., 10 minutes at 65° C., and 1 μl was transformed into E. coli DH5α.After expression, aliquots were spread on agar plates containing 200μg/ml kanamycin and incubated at 37° C.

Results, expressed as the number of colonies per 1 μl of recombinationreaction are presented in Table 5: TABLE 5 Vector Donor Gene DonorColonies Predicted % product Circular 1405 — 100 — Circular 1405Circular 1502 3740 3640/3740 = 97% Linear 1405 — 90 — Linear 1405Circular 1502 172,000 171,910/172,000 = 99.9% Circular 1405 Linear 150273,000 72,900/73,000 = 99.9% Circular 1603 — 80 — Circular 1603 Circular1706 410 330/410 = 80% Linear 1603 — 270 — Linear 1603 Circular 17067000 6730/7000 = 96% Circular 1603 Linear 1706 10,800 10,530/10,800 =97%

Analysis. In all configurations, i.e., circular or linear, thepEZC1405×pEZC1502 pair (with att sites in inverted repeat configuration)was more efficient than pEZC1603×pEZC1706 pair (with att sites mutatedto avoid hairpin formation). The pEZC1603×pEZC1706 pair gave higherbackgrounds and lower efficiencies than the pEZC1405×pEZC1502 pair.While less efficient, 80% or more of the colonies from thepEZC1603×pEZC1706 reactions were expected to contain the desired plasmidproduct. Making one partner linear stimulated the reactions in allcases.

Part III: Confirmation of Product Plasmids's Structure

Six colonies each from the linear pEZC1405 (FIG. 5G)×circular pEZC1502(FIG. 5H), circular pEZC1405×linear pEZC1502, linear pEZC1603 (FIG.6A)×circular pEZC1706 (FIG. 6B), and circular pEZC1603×linear pEZC1706reactions were picked into rich medium and miniprep DNAs were prepared.Diagnostic cuts with Ssp I gave the predicted restriction fragments forall 24 colonies.

Analysis. Recombination reactions between plasmids with mutant attL andattR sites on the same molecules gave the desired plasmid products witha high degree of specificity.

Example 5 Recombinational Cloning with a Toxic Gene

Part I: Background

Restriction enzyme Dpn I recognizes the sequence GATC and cuts thatsequence only if the A is methylated by the dam methylase. Most commonlyused E. coli strains are dam⁺. Expression of Dpn I in dam⁺ strains of E.coli is lethal because the chromosome of the cell is chopped into manypieces. However, in dam⁻ cells expression of Dpn I is innocuous becausethe chromosome is immune to Dpn I cutting.

In the general recombinational cloning scheme, in which the vector donorcontains two segments C and D separated by recombination sites,selection for the desired product depends upon selection for thepresence of segment D, and the absence of segment C. In the originalExample segment D contained a drug resistance gene (Km) that wasnegatively controlled by a repressor gene found on segment C. When C waspresent, cells containing D were not resistant to kanamycin because theresistance gene was turned off.

The Dpn I gene is an example of a toxic gene that can replace therepressor gene of the above embodiment. If segment C expresses the Dpn Igene product, transforming plasmid CD into a dam+ host kills the cell.If segment D is transferred to a new plasmid, for example byrecombinational cloning, then selecting for the drug marker will besuccessful because the toxic gene is no longer present.

Part II: Construction of a Vector Donor Using Dpn I as a Toxic Gene

The gene encoding Dpn I endonuclease was amplified by PCR using primers5′CCA CCA CAA ACG CGT CCA TGG AAT TAC ACT TTA ATT TAG3′ (SEQ. ID NO: 17)and 5′ CCA CCA CAA GTC GAC GCA TGC CGA CAG CCT TCC AAA TGT3′ (SEQ. IDNO: 18) and a plasmid containing the Dpn I gene (derived from plasmidsobtained from Sanford A. Lacks, Brookhaven National Laboratory, Upton,N.Y.; also available from American Type Culture Collection as ATCC67494) as the template.

Additional mutations were introduced into the B and B′ regions of attLand attR, respectively, by amplifying existing attL and attR domainswith primers containing the desired base changes. Recombination of themutant attL3 (made with oligo Xis115) and attR3 (made with oligo Xis112)yielded attB3 with the following sequence (differences from attB 1 inbold):   B           O         B′ A

CCC

GCTTT

TTGTACAAA

T

GT (SEQ. ID NO: 8) T

GGT CGAAA

AACATGTTT CA

CA (SEQ. ID NO: 35)The attL3 sequence was cloned in place of attL2 of an existing GeneDonor plasmid to give the plasmid pEZC2901 (FIG. 7A). The attR3 sequencewas cloned in place of attR2 in an existing Vector Donor plasmid to giveplasmid pEZC2913 (FIG. 7B) Dpn I gene was cloned into plasmid pEZC2913to replace the tet repressor gene. The resulting Vector Donor plasmidwas named pEZC3101 (FIG. 7C). When pEZC3101 was transformed into thedam⁻ strain SCS110 (Stratagene), hundreds of colonies resulted. When thesame plasmid was transformed into the dam+ strain DH5α, only one colonywas produced, even though the DH5α cells were about 20 fold morecompetent than the SCS110 cells. When a related plasmid that did notcontain the Dpn I gene was transformed into the same two cell lines, 28colonies were produced from the SCS110 cells, while 448 coloniesresulted from the DH5α cells. This is evidence that the Dpn I gene isbeing expressed on plasmid pEZC3101 (FIG. 7C), and that it is killingthe dam⁺ DH5α cells but not the dam⁻ SCS110 cells.III: Demonstration of Recombinational Cloning Using Dpn I Selection

A pair of plasmids was used to demonstrate recombinational cloning withselection for product dependent upon the toxic gene Dpn I. PlasmidpEZC3101 (FIG. 7C) was linearized with Mlu I and reacted with circularplasmid pEZC2901 (FIG. 7A). A second pair of plasmids using selectionbased on control of drug resistance by a repressor gene was used as acontrol: plasmid pEZC1802 (FIG. 7D) was linearized with Xba I andreacted with circular plasmid pEZC1502 (FIG. 5H). Eight microliterreactions containing the same buffer and proteins Xis, Int, and IHF asin previous examples were incubated for 45 minutes at 25° C., then 10minutes at 75° C., and 1 μl aliquots were transformed into DH5α (i.e.,dam+) competent cells, as presented in Table 6. TABLE 6 Reac- Basis oftion # Vector donor selection Gene donor Colonies 1 pEZC3101/Mlu Dpn Itoxicity — 3 2 pEZC3101/Mlu Dpn I toxicity Circular pEZC2901 4000 3pEZC1802/Xba Tet repressor — 0 4 pEZC1802/Xba Tet repressor CircularpEZC1502 12100

Miniprep DNAs were prepared from four colonies from reaction #2, and cutwith restriction enzyme Ssp I. All gave the predicted fragments.

Analysis: Subcloning using selection with a toxic gene was demonstrated.Plasmids of the predicted structure were produced.

Example 6 Cloning of Genes with Uracil DNA Glycosylase and Subcloning ofthe Genes with Recombinational Cloning to Make Fusion Proteins

Part I: Converting an Existing Expression Vector to a Vector Donor forRecombinational Cloning

A cassette useful for converting existing vectors into functional VectorDonors was made as follows. Plasmid pEZC3101 (FIG. 7C) was digested withApa I and Kpn I, treated with T4 DNA polymerase and dNTPs to render theends blunt, further digested with Sma I, Hpa I, and AlwN I to render theundesirable DNA fragments small, and the 2.6 kb cassette containing theattR1 —Cm^(R)—Dpn I—attR-3 domains was gel purified. The concentrationof the purified cassette was estimated to be about 75 ng DNA/μl.

Plasmid pGEX-2TK (FIG. 8A) (Pharmacia) allows fusions between theprotein glutathione S transferase and any second coding sequence thatcan be inserted in its multiple cloning site. pGEX-2TK DNA was digestedwith Sma I and treated with alkaline phosphatase. About 75 ng of theabove purified DNA cassette was ligated with about 100 ng of thepGEX-2TK vector for 2.5 hours in a 5 μl ligation, then 1 μl wastransformed into competent BRL 3056 cells (a dam⁻ derivative of DH10B;dam⁻ strains commercially available include DM1 from Life Technologies,Inc., and SCS 110 from Stratagene). Aliquots of the transformationmixture were plated on LB agar containing 100 μg/ml ampicillin(resistance gene present on pGEX-2TK) and 30 μg/ml chloramphenicol(resistance gene present on the DNA cassette). Colonies were picked andminiprep DNAs were made. The orientation of the cassette in pGEX-2TK wasdetermined by diagnostic cuts with EcoR I. A plasmid with the desiredorientation was named pEZC3501 (FIG. 8B).

Part II: Cloning Reporter Genes Into an Recombinational Cloning GeneDonor Plasmid in Three Reading Frames

Uracil DNA glycosylase (UDG) cloning is a method for cloning PCRamplification products into cloning vectors (U.S. Pat. No. 5,334,515,entirely incorporated herein by reference). Briefly, PCR amplificationof the desired DNA segment is performed with primers that contain uracilbases in place of thymidine bases in their 5′ ends. When such PCRproducts are incubated with the enzyme UDG, the uracil bases arespecifically removed. The loss of these bases weakens base pairing inthe ends of the PCR product DNA, and when incubated at a suitabletemperature (e.g., 37° C.), the ends of such products are largely singlestranded. If such incubations are done in the presence of linear cloningvectors containing protruding 3′ tails that are complementary to the 3′ends of the PCR products, base pairing efficiently anneals the PCRproducts to the cloning vector. When the annealed product is introducedinto E. coli cells by transformation, in vivo processes efficientlyconvert it into a recombinant plasmid.

UDG cloning vectors that enable cloning of any PCR product in all threereading frames were prepared from pEZC3201 (FIG. 8K) as follows. Eightoligonucleotides were obtained from Life Technologies, Inc. (all written5′→3′: rf1 top (GGCC GAT TAC GAT ATC CCA ACG ACC GAA AAC CTG TAT TTT CAGGGT) (SEQ. ID NO:19), rf1 bottom (CAG GTT TTC GGT CGT TGG GAT ATC GTAATC)(SEQ. ID NO:20), rf2 top (GGCCA GAT TAC GAT ATC CCA ACG ACC GAA AACCTG TAT TTT CAG GGT)(SEQ. ID NO:21), rf2 bottom (CAG GTT TTC GGT CGT TGGGAT ATC GTA ATC T)(SEQ. ID NO:22), rf3 top (GGCCAA GAT TAC GAT ATC CCAACG ACC GAA AAC CTG TAT TTT CAG GGT)(SEQ. ID NO:23), rf3 bottom (CAG GTTTTC GGT CGT TGG GAT ATC GTA ATC TT)(SEQ. ID NO:24), carboxy top (ACC GTTTAC GTG GAC)(SEQ. ID NO:25) and carboxy bottom (TCGA GTC CAC GTA AAC GGTTCC CAC TTA TTA)(SEQ. ID NO:26). The rf1, 2, and 3 top strands and thecarboxy bottom strand were phosphorylated on their 5′ ends with T4polynucleotide kinase, and then the complementary strands of each pairwere hybridized. Plasmid pEZC3201 (FIG. 8K) was cut with Not I and SalI, and aliquots of cut plasmid were mixed with the carboxy-oligo duplex(Sal I end) and either the rf1, rf2, or rf3 duplexes (Not I ends) (10 μgcut plasmid (about 5 pmol) mixed with 250 pmol carboxy oligo duplex,split into three 20 μl volumes, added 5 μl (250 pmol) of rf1, rf2, orrf3 duplex and 2 μl=2 units T4 DNA ligase to each reaction). After 90minutes of ligation at room temperature, each reaction was applied to apreparative agarose gel and the 2.1 kb vector bands were eluted anddissolved in 50 μl of TE.

Part III: PCR of CAT and phoA Genes

Primers were obtained from Life Technologies, Inc., to amplify thechloramphenicol acetyl transferase (CAT) gene from plasmid pACYC184, andphoA, the alkaline phosphatase gene from E. coli. The primers had12-base 5′ extensions containing uracil bases, so that treatment of PCRproducts with uracil DNA glycosylase (UDG) would weaken base pairing ateach end of the DNAs and allow the 3′ strands to anneal with theprotruding 3′ ends of the rf1, 2, and 3 vectors described above. Thesequences of the primers (all written 5′→3′) were: CAT left, UAU UUU CAGGGU ATG GAG AAA AAA ATC ACT GGA TAT ACC (SEQ. ID NO:27); CAT right, UCCCAC UUA UUA CGC CCC GCC CTG CCA CTC ATC (SEQ. ID NO:28); phoA left, UAUUUU CAG GGU ATG CCT GTT CTG GAA AAC CGG (SEQ. ID NO:29); and phoA right,UCC CAC UUA UUA TTT CAG CCC CAG GGC GGC TTT C (SEQ. ID NO:30). Theprimers were then used for PCR reactions using known method steps (see,e.g., U.S. Pat. No. 5,334,515, entirely incorporated herein byreference), and the polymerase chain reaction amplification productsobtained with these primers comprised the CAT or phoA genes with theinitiating ATGs but without any transcriptional signals. In addition,the uracil-containing sequences on the amino termini encoded thecleavage site for TEV protease (Life Technologies, Inc.), and those onthe carboxy terminal encoded consecutive TAA nonsense codons.

Unpurified PCR products (about 30 ng) were mixed with the gel purified,linear rf1, rf2, or rf3 cloning vectors (about 50 ng) in a 10 μlreaction containing 1×REact 4 buffer (LTI) and 1 unit UDG (LTI). After30 minutes at 37° C., 1 μl aliquots of each reaction were transformedinto competent E. coli DH5α cells (LTI) and plated on agar containing 50μg/ml kanamycin. Colonies were picked and analysis of miniprep DNAshowed that the CAT gene had been cloned in reading frame 1(pEZC3601)(FIG. 8C), reading frame 2 (pEZC3609)(FIG. 8D) and readingframe 3 (pEZC3617)(FIG. 8E), and that the phoA gene had been cloned inreading frame 1 (pEZC3606)(FIG. 8F), reading frame 2 (pEZC3613)(FIG. 8G)and reading frame 3 (pEZC3621)(FIG. 8H).

Part IV: Subcloning of CAT or phoA from UDG Cloning Vectors into a GSTFusion Vector

Plasmids encoding fusions between GST and either CAT or phoA in allthree reading frames were constructed by recombinational cloning asfollows. Miniprep DNA of GST vector donor pEZC3501(FIG. 8B) (derivedfrom Pharmacia plasmid pGEX-2TK as described above) was linearized withCla I. About 5 ng of vector donor were mixed with about 10 ng each ofthe appropriate circular gene donor vectors containing CAT or phoA in 8μl reactions containing buffer and recombination proteins Int, Xis, andIHF (above). After incubation, 1 μl of each reaction was transformedinto E. coli strain DH5α and plated on ampicillin, as presented in Table7. TABLE 7 Colonies (10% of each DNA transformation) Linear vector donor(pEZC3501/Cla) 0 Vector donor + CAT rf1 110 Vector donor + CAT rf2 71Vector donor + CAT rf3 148 Vector donor + phoA rf1 121 Vector donor +phoA rf2 128 Vector donor + phoA rf3 31Part V: Expression of Fusion Proteins

Two colonies from each transformation were picked into 2 ml of richmedium (CIRCLEGROW® brand culture medium, Bio101 Inc.) in 17×100 mmplastic tubes (FALCON® brand plasticware, Cat. No. 2059, BectonDickinson) containing 100 μg/ml ampicillin and shaken vigorously forabout 4 hours at 37° C., at which time the cultures were visibly turbid.One ml of each culture was transferred to a new tube containing 10 μl of10% (w/v) IPTG to induce expression of GST. After 2 hours additionalincubation, all cultures had about the same turbidity; the A600 of oneculture was 1.5. Cells from 0.35 ml each culture were harvested andtreated with sample buffer (containing SDS and β-mercaptoethanol) andaliquots equivalent to about 0.15 A600 units of cells were applied to aNovex 4-20% gradient polyacrylamide gel. Following electrophoresis thegel was stained with Coomassie blue.

Results: Enhanced expression of single protein bands was seen for all 12cultures. The observed sizes of these proteins correlated well with thesizes predicted for GST being fused (through attB recombination siteswithout stop codons) to CAT or phoA in three reading frames: CAT rf1=269amino acids; CAT rf2=303 amino acids; CAT rf3=478 amino acids; phoArf1=282 amino acids; phoA rf2=280 amino acids; and phoA rf3=705 aminoacids.

Analysis: Both CAT and phoA genes were subcloned into a GST fusionvector in all three reading frames, and expression of the six fusionproteins was demonstrated.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be appreciated by oneskilled in the art from a reading of this disclosure that variouschanges in form and detail can be made without departing from the truescope of the invention and appended claims. All patents and publicationscited herein are entirely incorporated herein by reference.

1. A method for synthesizing a double stranded nucleic acid moleculecomprising: (a) mixing one or more nucleic acid templates with apolypeptide having polymerase activity and one or more primerscomprising at least a first recombination site or portions thereof; (b)incubating said mixture under conditions sufficient to synthesize afirst nucleic acid molecule which is complementary to all or a portionof said one or more templates and which comprises at least said firstrecombination site or portions thereof; and (c) incubating said firstnucleic acid molecule in the presence of one or more primers comprisingat least a second recombination site or portions thereof underconditions sufficient to synthesize a second nucleic acid moleculecomplementary to all or a portion of said first nucleic acid molecule,thereby producing a double stranded nucleic acid molecule comprising atleast said first and second recombination sites or portions thereof,wherein at least one of said first and second recombination sitescomprises one or more mutations that remove one or more stop codons fromsaid recombination sites.
 2. A method for synthesizing a double strandednucleic acid molecule comprising: (a) mixing one or more nucleic acidtemplates with a polypeptide having polymerase activity and one or moreprimers comprising at least a first recombination site or portionsthereof; (b) incubating said mixture under conditions sufficient tosynthesize a first nucleic acid molecule which is complementary to allor a portion of said one or more templates and which comprises at leastsaid first recombination site or portions thereof; and (c) incubatingsaid first nucleic acid molecule in the presence of one or more primerscomprising at least a second recombination site or portions thereofunder conditions sufficient to synthesize a second nucleic acid moleculecomplementary to all or a portion of said first nucleic acid molecule,thereby producing a double stranded nucleic acid molecule comprising atleast said first and second recombination sites or portions thereof,wherein at least one of said first and second recombination sitescomprises one or more mutations that avoids hairpin formation in saidrecombination sites.
 3. A method for synthesizing a double strandednucleic acid molecule comprising: (a) mixing one or more nucleic acidtemplates with a polypeptide having polymerase activity and one or moreprimers comprising at least a first recombination site or portionsthereof; (b) incubating said mixture under conditions sufficient tosynthesize a first nucleic acid molecule which is complementary to allor a portion of said one or more templates and which comprises at leastsaid first recombination site or portions thereof; and (c) incubatingsaid first nucleic acid molecule in the presence of one or more primerscomprising at least a second recombination site or portions thereofunder conditions sufficient to synthesize a second nucleic acid moleculecomplementary to all or a portion of said first nucleic acid molecule,thereby producing a double stranded nucleic acid molecule comprising atleast said first and second recombination sites or portions thereof,wherein at least one of said first and second recombination sitescomprises at least one nucleic acid sequence selected from the groupconsisting of SEQ ID NOs: 1-16 or a DNA sequence complementary thereto.4. The method of claim 1, wherein said recombination sites or portionsthereof are located at or near one terminus of said double strandednucleic acid molecule.
 5. The method of claim 2, wherein saidrecombination sites or portions thereof are located at or near oneterminus of said double stranded nucleic acid molecule.
 6. The method ofclaim 3, wherein said recombination sites or portions thereof arelocated at or near one terminus of said double stranded nucleic acidmolecule.
 7. The method of claim 1, wherein said first or secondrecombination sites are selected from the group consisting of attBsites, attP sites, attL sites, attR sites, lox sites, and portionsthereof.
 8. The method of claim 2, wherein said first or secondrecombination sites are selected from the group consisting of attBsites, attP sites, attL sites, attR sites, lox sites, and portionsthereof.
 9. The method of claim 3, wherein said first or secondrecombination sites are selected from the group consisting of attBsites, attP sites, attL sites, attR sites, lox sites, and portionsthereof.
 10. The method of claim 1, further comprising amplifying saidfirst and second nucleic acid molecules.
 11. The method of claim 2,further comprising amplifying said first and second nucleic acidmolecules.
 12. The method of claim 3, further comprising amplifying saidfirst and second nucleic acid molecules.
 13. The method of claim 1,wherein said recombination sites or portions thereof are located at ornear one or both termini of said double stranded nucleic acid molecule.14. The method of claim 2, wherein said recombination sites or portionsthereof are located at or near one or both termini of said doublestranded nucleic acid molecule.
 15. The method of claim 3, wherein saidrecombination sites or portions thereof are located at or near one orboth termini of said double stranded nucleic acid molecule.
 16. Themethod of claim 1, wherein said first and second recombination sites donot recombine with each other.
 17. The method of claim 2, wherein saidfirst and second recombination sites do not recombine with each other.18. The method of claim 3, wherein said first and second recombinationsites do not recombine with each other.
 19. A method for synthesizing adouble stranded nucleic acid molecule comprising: (a) mixing one or morenucleic acid templates with a polypeptide having polymerase activity andone or more primers comprising at least a first recombination site orportions thereof; (b) incubating said mixture under conditionssufficient to synthesize a first nucleic acid molecule which iscomplementary to all or a portion of said one or more templates andwhich comprises at least said first recombination site or portionsthereof; and (c) incubating said first nucleic acid molecule in thepresence of one or more primers comprising at least a secondrecombination site or portions thereof under conditions sufficient tosynthesize a second nucleic acid molecule complementary to all or aportion of said first nucleic acid molecule, thereby producing a doublestranded nucleic acid molecule comprising at least said first and secondrecombination sites or portions thereof, wherein at least one of saidfirst and second recombination sites comprises at least one nucleotidesequence that has at least 80-99% homology to a nucleotide sequenceselected from the group of sequences consisting of SEQ ID NOs: 1-16, anda corresponding or complementary DNA or RNA sequence.
 20. The method ofclaim 19, wherein said recombination sites or portions thereof arelocated at or near one terminus of said double stranded nucleic acidmolecule.
 21. The method of claim 19, further comprising amplifying saidfirst and second nucleic acid molecules.
 22. The method of claim 19,wherein said recombination sites or portions thereof are located at ornear one or both termini of said double stranded nucleic acid molecule.23. The method of claim 19, wherein said first and second recombinationsites do not recombine with each other.